Research On Ti(C,N)-based Cermets With High Strength,Toughness, And Corrosion Resistance

Ti(C,N)-based cermet has been attracting more attention because of its high-temperature hardness, excellent wear resistance, perfect physical and chemical stability, very low friction coefficient to metals and superior high-temperature oxidation resistance, which make it useful in a wide range of applications such as the high-speed dry milling tool fields and wear-resistant parts fields. As the dry milling tools, the cermets suitable for high-speed finishing and semi-finishing fields of steel, belong to a new hard tool material. Thus, excellent comprehensive mechanical properties and superior corrosion resistance performance are the key to meet different service environment for Ti(C,N)-based cermets.Using solid solution type TiC0.7N0.3and interstitial phase type TiC and TiN as the raw materials, and using different Mo2C content as additive, the five different component Ti(C,N)-based cermets prepared by sinter-HIP at the temperature of1440℃under a pressure of6MPa, were investigated. The microstructures and mechanical properties were observed and tested, and the electrochemical behaviors were investigated in H2SO4(pH=1), Na2SO4(pH=7) and NaOH (pH=13) solutions at25℃utilizing potentiodynamic polarization, electrochemical impedance spectroscopy and surface analytical techniques. At the same time, oxidation behaviors were studied at the temperature of800℃for16h, In addition, the corrosion mechanism and the oxidation mechanism of Ti(C,N)-based cermets were discussed. The main results are summarized as following:1. Although there are differences in the relative contents of the chemical components of the hard phase and binder phase, the investigated cermets have the same phase composition. The hard phase is a cubic structured single-phase carbonitride containing Ti, W, Mo, Ta and Cr, and the binder phase is a cubic structured Ni-Co based solid solution. However, for the cermet prepared from TiC0.7N0.3, it is characterized by a lower Ti and Ta content and a lower solid solubility in the binder phase, a homogeneous microstructure, a finer grain size, a more regular and stable core-rim structure, a well coated core, a thinner rim, and accordingly much better mechanical properties. In addition, it is characterized by a homogeneous microstructure, a finer grain size, a more regular and stable core-rim structure and accordingly much better mechanical properties at the Mo2C content. The differences in the microstructure and properties are explained in terms of the accelerant-linkage effect related with the binder phase and the degree of order in reaction-diffusion during the reaction-sintering process.2. The results of electrochemical experiments show that, the cermet prepared from TiC0.7NN0.3, possesses superior corrosion resistance than that prepared from TiC and TiN in these three corrosion solution. And Mo2C additive shows bad effect on the improving corrosion resistance of cermet in these three corrosion solution.3. The electrochemical corrosion mechanism of Ti(C,N)-based cermets is as following:the potential difference between hard phase and binder phase leads to preferential corrosion and active dissolution of Ni-Co based binder phase; and the dissolution rate of binder phase in neutral and alkaline solution decrease compared with that in acid solution; after corrosion of binder phase, hard phase skeleton exposes to the corrosion solution; this causes oxidation reaction of part of hard phase, and the oxidation amount of hard phase in acid solution is more than that in alkaline solution, so, the corrosion resistance of alloy in alkaline solution is better than that in acid solution.4. The results of high temperature oxidation experiment and oxidation surface analyses show that, continuous and compact oxidation film forms on the surfaces of all these investigated cermets, and the cermets are characterized by multilayer structure after oxidated. That is reaction layer, intermediate reaction layer and unreaction matrix. However, the cermet prepared from TiC0.7N0.3, possesses superior oxidation resistance than that prepared from TiC and TiN, and Mo2C additive shows a bad effect on the improving oxidation resistance of cermet.5. The oxidation mechanism of Ti(C,N)-based cermets is as following:the oxidation behavior of Ti(C,N)-based cermets belongs to "passivation oxidation". Oxidation mechanism is characterized by the diffusion reaction of oxygen from the exterior to the interior. Ni, Co, Cr and Ti elements possessing good oxytropism, migrate to the surface, and preferentially form oxidation product films that riched in Ni, Co, Cr and Ti elements. The oxide films prevent external oxygen atoms migrating to internal and internal alloy elements migrating to external.